1. Field of the Invention
The present invention relates to a plastic sheet manufacturing method, and to a plastic sheet manufacturing apparatus that laminates films which transmit light, and in which an image is directly formed (recorded) by an electrophotographic image forming device. More specifically, the invention relates to a manufacturing method, and to a manufacturing apparatus of plastic sheets used as information media containing personal information and/or image information which is read out by either a non-contact or contact reading method. Examples of applications are documents containing a facial portrait such as cash cards, employment identity cards, student identity cards, personal membership cards, residency certificates, various types of driving licenses, and various certificates of qualifications. Other applications, in the medical field, are for purposes of identification, such as image sheets, image display boards and display labels.
2. Description of the Related Art
In recent years, along with the development of image forming techniques, means have been developed of forming a considerable number of images of a uniform quality at a low price by using various printing methods such as intaglio printing, relief printing, planographic printing, gravure printing, and screen printing. These printing methods are also often used in surface printing of information media which store predetermined information, and which are communicable with external devices in either a contact or non-contact manner. Examples of such media are integrated circuit cards, magnetic cards, optical cards and cards in which at least two of these categories of card are combined.
However, screen printing, for example, requires substantial numbers of printing plates, of a quantity identical to the number of images to be printed, and, in the case of color printing, screen printing further requires printing plates of a quantity identical to the number of colors to be used in each image. Accordingly, these printing methods are unsuitable for printing of personal identification information on an individual basis (e.g., facial portraits, names, addresses, dates of birth, and various types of license).
The image forming method which is generally used at the present time to print such information is one in which printers print images by a sublimation type thermal transfer system using an ink ribbon, or by a fusion type thermal transfer system. By using such printing methods, personal identification information can be easily printed. However, when a printing speed is increased, the quality of resolution is diminished. On the other hand, when the quality of resolution is enhanced, the printing speed is reduced.
In contrast, in an image forming (printing) method using an electrophotographic system, the surface of an image holding member is electrostatically and uniformly charged, and exposed to light according to an image signal. An electrostatic latent image is formed by the difference in electric potential between exposed portions and unexposed portions, and then electrostatically developed with colored powder (image forming material), called a toner, and having a polarity opposite to, (or the same as), that of the charged portions of the image holding member. As a result, a visible image (toner image) is formed on the image holding member surface. In circumstances when a color image is formed, these steps can be repeated several times, or alternatively image forming units can be arranged in parallel so as to form visible color images. The visible color images are transferred and fixed onto an image recording medium so that a full color image is obtained. In this context, fixing means melting the colored powder, mainly by heat, and then cooling it.
Since, as mentioned above, in the electrophotographic system, the electrostatic latent image is electrically formed on the image holding member surface according to an image signal, identical images can be repeatedly formed, and, as and when different images are required, the can also be easily formed. Further, the toner image on the image holding member surface can be almost completely transferred onto the surface of the image recording medium, and a minute amount of toner remaining on the image holding member surface can be easily removed by a resin blade, or by a brush. Accordingly, printed materials for multi-product small-batch production can easily be manufactured.
The toner is usually formed by melting and mixing a hot-melt resin and a pigment, together with on occasions optional additives such as a charge control agent, and then grinding the resultant kneaded matter into particles. Further, an electrostatic latent image obtained by means of the electrophotographic system has a much higher degree of resolution than the particulate toner, and thus a satisfactory degree of resolution can be expected even when compared with the degree of resolution obtained both in screen printing and in thermal transfer using an ink ribbon.
As for color images, color toners having four primary colors, namely cyan, magenta, yellow and black, can be used. By mixing these toners, colors similar to those obtained by printing can theoretically be reproduced. As for the color toners, since a toner resin and a pigment can be mixed comparatively freely, an image masking property of the toners can easily be enhanced.
Examples where various cards printed by use of the electrophotographic device described above include the following.
One method has been suggested in Japanese Patent Application Laid-Open (JP-A) No. 2001-92255 in which not only various kinds of personal information but also an invisible bar code are printed by means of the electrophotographic method on a polyvinyl chloride sheet of a thickness of 250 μm, or on a polyester sheet of a thickness of 280 μm, and a film is then laminated on the printed surface of the sheet by a hot-press machine.
However, since a friction coefficient between the sheets is too extreme, and the sheets accordingly adhere to each other, the conveyance properties of the sheets are poor and the electrophotographic device may on occasions stop. Moreover, it is difficult to transfer satisfactorily an image forming material (toner) onto an insulator (sheet) of a thickness of 250 μm or more, and on occasions defects in images may increase. Further, when an attempt is made to use an electrophotographic device to print an image on a resin film which is softened at a comparatively low temperature, a fixing temperature in a fixing step can be higher than the softening temperature of the film, and therefore the film may soften, and may adhere to and wind around a fixing unit, causing jamming. Moreover, the image forming material may offset onto the fixing unit, and furthermore, when fixing of a sheet of a thickness of 250 μm is continued, an edge (corner) of the sheet may damage the fixing unit excessively.
It has also been suggested that personal identification information be printed on a sheet which transmits light, and that printing be executed by using a reflected image (for example, see JP-A No. 11-334265). However, as regards the sheet which transmits light, this application merely discloses that at least a part of the sheet is preferably a biaxially oriented polyester film, or a film in which the biaxially oriented polyester film is combined with a film made of ABS or polyester, but may be polyvinyl chloride.
Therefore, since the film in this specification is simply an insulator, defects occur during the transfer of the image forming material onto the film surface, and thus it is not possible to secure a degree of resolution equivalent to that obtainable with a thermal transfer system. Further, the laminate sheet used in this device, in which emphasis is placed on an improvement in productivity, is in the form of a roll. Therefore, in order to cope with an emergency or with multi-product production in which different data for different persons needs to be printed on cards, considerable loss and waste may occur.
Furthermore, consideration has barely yet been given to the question of automatizing not only the image forming onto the film which transmits light but also the steps of transporting and laminating the film, which transmits light and onto which an image has been formed, and a core sheet (base material). From the viewpoint of improving productivity, it is necessary to re-appraise the above steps and to design a manufacturing apparatus.
With regard to a manufacturing apparatus for automatizing the laminating step, for example, a device has been suggested in which a pair of upper and lower rollers on an inlet side and a pair of upper and lower rollers on an outlet side are held up in parallel, and an endless belt is wound around at least the lower roller on the inlet side and the lower roller on the outlet side, and the rollers on the inlet side are used as a heating unit and the rollers on the outlet side are used as a cooling unit (for example, see JP-A No. 2-25331.
However, from the viewpoint of costs, the endless belt used in the above apparatus is generally formed by welding a metal plate of stainless steel. On such an endless belt, the welded joint portion is not level with other portions. When the uneven joint portion is opposed to the plastic sheet manufactured at the time of lamination, a trace of the uneven joint portion remains on the plastic sheet manufactured, and thus this sheet can turn into a defective product.
Accordingly, there is a need for a plastic sheet manufacturing method and a plastic sheet manufacturing apparatus with which plastic sheets of a high degree of resolution, can be consistently manufactured to a high level of productivity, without producing defective products.
There is also a need for a plastic sheet manufacturing apparatus which, even when used together with an image forming device which is not significantly different from a conventional electrophotographic device, can consistently manufacture plastic sheets of a high degree of resolution to a high level of productivity, without producing defective products.